Atomic layer epitaxy (ALE) of metal oxides traditionally reacts a metal chloride precursor such as TiCl.sub.4 with water to form a metal oxide layer. One such method is described, for example, in "Atomic Layer Epitaxy", by T. Suntola & M. Simpson (Chapman & Hall, 1990, New York, N.Y. U.S. Pat. No. 5,256,244 to Ackerman discloses the use of a hydrolyzable chloride of a metal and water to deposit a metal oxide film by ALE.
These prior art methods suffer from the disadvantage of residual chlorine being retained in the oxide film as well as possible formation of chlorides. As is well known to those skilled in the art, chlorine and chlorides are not desirable in electronic devices. This is because the resultant metal oxide formed from metal chloride precursors will be slightly conductive (high leakage current) due to ionic Cl.sup.- conduction. Moreover, chlorine can diffuse out of the metal oxide and adversely affect other regions of the electronic device.
U.S. Pat. No. 4,058,430 to Suntola discloses the use of a single vapor element reacted with a substance to form a film. This method suffers the following disadvantages: the need to physically evaporate the element; the low volatility of some elements and; in the case of metal oxide films, a need to oxidize the element.
U.S. Pat. No. 4,861,417 to Mochizuki, et al. discloses the use of an alkyl aluminum precursor such as trimethylaluminum or triethylaluminum as the source material for the growth of III-V compound semiconductors by ALE. U.S. Pat. No. 5,166,092 to Mochizuki, et al., on the other hand, discloses the use of different polarites and source compounds containing at least one methyl group for the growth of III-V compound semiconductors by ALE. These two prior art methods suffer from the disadvantage that residual carbon may be into the films resulting from incomplete decomposition of the alkylated aluminum precursors. Moreover, carbon is not desirable because it can make the metal oxide slightly conductive, and carbon can also diffuse out of the metal oxide and adversely affect other regions of the electronic device.
Another technique that is commonly employed in forming metal films is Molecular Beam Epitaxy (MBE). MBE traditionally uses an evaporated metal as a source material. Variations of MBE include gas source MBE, organometallic MBE, metal organic MBE and chemical beam epitaxy which typically utilize metal carbonates, alkoxides, .beta.-diketonates or halides as the source material.
Anhydrous volatile metal nitrates without the use of oxidizing co-reactants have been described as precursors for chemical vapor deposition (CVD) of metal oxide films, see W. L. Gladfelter, et al. "Anhydrous Metal Nitrates as Volatile Single Source Precursors for the CVD of Metal Oxide Films", Chem. Vap. Dep. 1998, 4, No. 6, p.220 and references cited therein. The use of metal nitrate precursors in CVD applications is also disclosed in W.L Gladfelter, et al., "Low Temperature CVD of Crystalline Titanium Dioxide Films Using Tetranitratotitanium(iv)", Chem. Vap. Dep. 1998, 4, No. 1, p.9 and W. L Gladfelter, et al. "Does Chemistry Really Matter in the Chemical Vapor Deposition of Titanium Dioxide? Precursor and Kinetic Effects on the Microstructure of Polycrystalline Films", JACS, 1999, 121, p.5220. Each of these references disclose that metal nitrate-containing precursors can be used in forming oxide films by CVD without the use of an oxidizing co-reactant. There is no disclosure in these references however of using the nitrate-containing precursors in ALD applications with oxidizing, nitriding or reducing co-reactants.
The present invention differs from the above cited references in that metal nitrate-containing precursors are used in conjunction with oxidizing, nitriding and reducing co-reactants to grow oxide, nitride and metal films, respectively by ALD.
In the above cited references, metal oxide films are grown by CVD without an oxidizing co-reactant. This is necessary because, as described in the references, the metal nitrate-containing precursors are powerful oxidizing and nitrating agents, capable of reacting vigorously with many compounds. In addition, the metal nitrate precursors readily decompose in the presence of ambient air, water, light and/or at temperatures as low as 100.degree. C. The reactivity of the metal nitrates makes gas phase reactions with other precursors, and with co-reactants such as oxidizing, nitriding and reducing agents likely. The gas phase reactions can lead to premature decomposition of the metal nitrate precursor, and of the co-reactants, potentially resulting in particle formation in the gas phase, reduced incorporation of reactants into the film, difficulty in reproducibility of film stoichiometry, thickness and uniformity across the wafer and contamination of delivery lines and CVD chamber due to uncontrolled decomposition. For example to form a nitride film, it is necessary to introduce a nitriding reagent such as ammonia into the gas stream. Ammonia can react with the metal nitrate precursor in the gas phase leading to premature decomposition in the gas phase and poor growth properties.
As described above, use of metal nitrates in a conventional CVD system has significant problems that may prevent success. However, use of the metal nitrate-containing precursors using ALD techniques would avoid the difficulties listed above. By alternating reactants in the gas stream, the opportunity for gas reactions is minimized allowing the use of metal nitrates with incompatible reagents such as oxidizing, nitriding, reducing agents, and other metal-containing precursors to form multicomponent metal oxide, metal nitride and metal films which can not be made by conventional CVD.
ALD differs from CVD and therefore has different precursor requirements than CVD. ALD is performed in a cyclic fashion with sequential alternating pulses of precursor, reactant and purge gas. The ALD precursor must have a self-limiting effect such that the precursor is adsorbed on the substrate up to monolayer. Because of the self-limiting effect, only one monolayer or sub-monolayer is deposited per operation cycle, and additional precursor will not be deposited on the grown layer even when excess precursor is supplied. In CVD, precursor and reactants arrive at the substrate simultaneously with film growth resulting from continuous chemical reactions of precursors on the substrate surface. Uniform and reproducible growth of the film is dependent on maintenance of the correct precursor and reactant flux at the substrate. The growth rate is proportional to the precursor flux at the substrate and to the substrate temperature. Because of the different growth mechanisms, the precursor requirements differ for ALD and CVD. In ALD, the precursor must readily adsorb at bonding sites on the growth surface in a self-limiting mode, and once adsorbed must readily react with co-reactant to form the desired film. In CVD, the precursor and the co-reactants must react appropriately at the substrate surface simultaneously to form the desired film. Thus, many useful CVD precursors are not viable as ALD precursors, and it is not trivial or obvious to select a precursor for the ALD method.